A Metalloprotein Inspired Ruthenium Complex ... - Wiley Online Library

DOI: 10.1002/slct.201600751

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A Metalloprotein Inspired Ruthenium Complex as an Efficient and Reusable Catalyst for Selective Oxidation of Alcohols to their Corresponding Carbonyl Compounds Ramen Jamatia,[a] Ajay Gupta,[a] Mrityunjoy Mahato,[b] Ranjit A. Patil,[c] Yuan-Ron Ma,[c] and Amarta K. Pal*[a] Metalloprotein inspired self-assembled ruthenium catalyst (3) was prepared from a linear amphiphilic polymer poly(Nisopropylacrylamide-co-N-vinylimidazole) (1) and ruthenium trichloride (2) via coordinative convolution. The ruthenium catalyst (3) was characterized using TEM, SEM, EDX, Powder XRD, TGA and XPS analysis. The polymer (1) was analysed using various characterization techniques such as FT-IR, 1H and 13C NMR and GPC analysis. The prepared ruthenium catalyst (3) was highly stable, globular, highly active and reusable. The ruthenium catalyst in 863 mol ppm level (with respect to

Introduction Metalloprotiens are supramolecular composites of polymeric peptides and metal species which efficiently catalyze many biological transformation reactions.[1, 2] The metallic species are bound to the imidazole unit of histidine within a supramolecular array of proteins. Therefore imidazole unit is used to mimic the artificial metalloprotein. Metalloprotein inspired metal catalyst ensures high catalytic activity (in ppm level), reusability and stability.[3] Therefore the design and development of metalloprotein inspired self-assembled polymeric metal catalyst has emerged as an interesting and sustainable field in organic, inorganic, supramolecular chemistry as well as industrial chemistry.[4] Recently Yamada and co-workers reported the preparation of self-assembled polymeric copper and palladium catalyst via the molecular convolution method.[5] We therefore decided to apply this concept for the preparation of self-assembled polymeric

[a] R. Jamatia, A. Gupta, Dr. A. K. Pal Department of Chemistry, Centre for Advanced Studies North-Eastern Hill University Shillong-793022, India E-mail: [email protected] [b] Dr. M. Mahato Department of Basic Sciences and Social Sciences, School of North-Eastern Hill University Shillong-793022, India [c] Dr. R. A. Patil, Prof. Y.-R. Ma Department Physics National Dong Hwa University Hualien-97401, Taiwan Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/slct.201600751

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ruthenium) could catalyse the selective oxidation of various alcohol substrates efficiently within a short period of time. The catalyst could be separated by simple filtration and reused for another five consecutive runs without substantial loss in activity of catalyst or yield of product. The Turn over number (TON) and Turn over frequency (TOF) of the catalyst reached 1031 and 516 h-1 respectively. An ICP-AES analysis of the organic phase of the reaction revealed insignificant (0.709 ppm of Ru) leaching of the catalyst.

ruthenium catalyst which would offer high catalytic activity with reusability for the selective oxidation of alcohols. Oxidation of alcohols to their corresponding carbonyls serves as a powerful functional group transformation reaction in organic synthesis. These transformation reactions results in carbonyl products which are versatile intermediates for various pharmaceutical, biological and agriculturally relevant compounds.[6] Stereoselective oxidation of alcohols to aldehydes is a challenging task to synthetic chemist. Traditionally, these processes have been achieved using Dess-Martin reagents,[7] Swern reagents[8] and chromium and manganese oxides.[9] These reagents however features several drawbacks such as the use of stoichiometric amount of the heavy metal complexes, difficulty to handle and hazardous waste disposal. In view of the recent environmental issues, various oxidation methods have been developed employing transition metals assisted by co-oxidants.[10] These protocols although brings a significant improvement. Most of the transition metal used are Pd, Pt, Au and Ir which are expensive metals.[11] Further the co-oxidant used are usually expensive reagents such as TEMPO and Ag2O or molecular oxygen which is difficult to handle.[10, 12] Among the transition metals, ruthenium based catalyst have been known to show excellent activity for various reactions such as isomerisation,[13] hydrogenation,[14] olefin metathesis[15] and oxidation[16] etc. Although, the previous Ru catalysed oxidation of alcohols were advantageous but they suffer from several drawbacks such as higher catalytic loading of Ru,[16h] use of toxic or expensive ligand additive,[16d] trimetallic catalyst containing expensive metal salts,[16g] longer reaction time,[16j] use of base etc.[16j] In light of the previous reports where ruthenium metal complexes served as a relatively inexpensive and efficient catalytic system with several aforementioned 5929

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Full Papers drawbacks, we therefore decided to focus our research towards the development of oxidation process using metalloprotein inspired ruthenium catalyst.

Results and Discussion According to Yamada and co-workers; linear amphiphilic polymer poly(N-isopropylacrylamide-co-N-vinylimidazole) (1) co-ordinate Pd2 + and Cu2 + via imidazole unit of the polymer. [5] We adopted the same mechanism for the synthesis of our ruthenium catalyst (3). The metallopreotein inspired ruthenium catalyst (3) was prepared via a molecular convolution method from a linear amphiphilic polymer poly(N-isopropylacrylamideco-N-vinylimidazole) (1) and ruthenium trichloride (2). The linear amphiphilic polymer poly(N-isopropylacrylamide-co-Nvinylimidazole) (1) was prepared according to the procedure given by Yamada et al.[5] To a solution of imidazole polymer (1, 1 g) in 10 mL of CHCl3 was added an aqueous solution of ruthenium trichloride (2, 157 mg) at room temperature. Upon addition of ruthenium trichloride, black precipitate was formed and the resulting black suspension was heated at 70 oC. After completion of the reaction (12 h) the black precipitate was filtered, washed with CHCl3 (3 X 10 mL) and water (3 X 10 mL) on the glass filter and dried under reduced pressure (Scheme 1)

Scheme 1. Synthetic representation of the metalloprotein inspired Ru catalyst. .

The synthesized polymeric ruthenium catalyst was highly stable and insoluble in most of the common solvents such as H2O, CHCl3, EtOH, THF and MeOH which makes it an attractive candidate for easy separation and reusability. Gel permeation chromatography (GPC) analysis of the prepared imidazole polymer was performed. The GPC chromatogram is given in S.I. Figure 2. From GPC data, it was established that the number average molecular weight (Mn) = 569 g/mol and the weight average molecular weight (Mw) = 616 g/mol. The GPC analysis provided the polydispersity index (PDI, Mw/Mn = 1.08) which indicates a narrow molecular weight distribution for the prepared polymer 1. The structure, morphology and composition of the synthesized polymeric ruthenium catalyst (3) were characterized by various techniques. In the FT-IR spectrum, peaks at 1642 and 1552 cm 1 corresponds to C=C and C=N ChemistrySelect 2016, 1, 5929 – 5935

stretching frequency of imidazole ring (S.I. Figure 1B). The absorption bands observed at 284 and 313 cm 1 is due to the Ru Cl stretching frequency. The peaks at 229 cm 1 can be attributed to Ru N stretching (S.I. Figure 1C), which is very close to the reported value.[17] The transmission electron microscope (TEM) (Figure-1 A) image, the selected area electron diffraction (SAED) pattern (Figure-1B) along with scanning electron microscope (SEM) image (Figure-1D) and energy dispersive X-ray (EDX) data (Figure-1C) supports the formation of polymer supported Ru compound and in its nano form as indicated by the arrow in the respective figures. The TEM image shows the distributed spherical nanoparticle with size ~ 10 nm. The EDX peak confirms the presence of Ru with ~ 7-9 % (weight) which is in line with the atomic emission spectroscopic (ICPAES) report (4.36 %). There is also similar report of Ru content in another polymer supported Ru catalyst.[18] The ring like SAED pattern (Figure-1B) is indicative that the Ru complex was semicrystalline in nature which is evidenced from earlier reports.[19] The low resolution SEM image (Figure-1D) is in line with the earlier report, which is much similar to the aggregate form of proteins.[20] Powder X-ray diffraction (XRD) data provide support for the structural aspect of Ru-Complex (Figure-2). There is a prominent characteristic peak observed at 21.310 (2q) for plane (130) along with some other peaks shown in S.I. Table-1. The lattice spacing (d-values) and the diffraction planes (hkl) were compiled from the literature data.[21, 22] It has been found that the peak position were shifted within 1 or 2 degree which may be due to the presence of polymer support in the Ru complex. The semicrystalline/amorphous like structure is evidenced from the background profile of the XRD data. In this regard it is also evident from literature that the Ru (III) complex has amorphous like characteristic compared to the other Pd (II) and Zr (IV) complex.[23] Thermal stability of the Ru complex was investigated using thermogravimetric analysis (TGA) over the temperature range 30–740 0C. The TGA data shows characteristics weight loss in roughly three steps (Figure 3A). The Ru complex was thermally stable up to a considerably higher temperature up to around 300 0C where there is a very little amount of weight loss ~ 10 %. The major weight loss was observed at about 380 0C to 460 0C, representing the characteristics decomposition and degradation transition with a weight loss ~ 97 %. In last step about 460– 740 0C, there is a total weight loss ~ 99.9 %. This is in agreement with the earlier results of another polymer supported Ru catalyst.[18] Thus the characteristic weight loss in TGA data is a complementary evidence for polymer supported Ru complex. The X-ray photoelectron spectroscopic (XPS) measurement was carried out to gain insight on the presence of Ru and the interacting species present on the surface of Ru-complex (Figure-3B). The core level XP spectrum was taken in the energy region 280–290 eV. The characteristic peak has been deconvoluted into five components for best fitting parameter of R2 value 0.999 using Gaussian multipeak fitting technique. The characteristic peak for Ru 3d5/2 and 3d3/2 was observed at 282.2 eV and 283.9 eV respectively indicating the presence of Ru.[24] Since the C1 s reference peak also lies at 285 eV it 5930


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Figure 1. (A, B) TEM images and SAED pattern of the nanoparticle of Ru-polymer complex obtained from TEM. (C, D) EDX data and SEM image of the RuPolymer complex nanoparticle. Arrows on the images are indicating the presence of Ru and its nanoparticle/microparticle.

overlaps with the Ru peak and somehow it reflects a little bit shifts in the deconvoluted data. The appearance of 3d5/2 peak at 282.2 eV is indicating Ru N species in + 3 oxidation state.[24] The 286.5 eV peak attributed to the Ru C species adjacent to carboxyl and 287.6 eV corresponds to the electron deficient carbon atom within the carboxyl.[25] Literature reports[16] suggest ruthenium metal to be an efficient catalyst for oxidation process. Therefore after preparation of the self-assembled polymeric ruthenium catalyst (3), we next proceeded to evaluate the catalytic activity of 3 for the selective oxidation of alcohols. Initially the reaction process was optimized using oxidation of benzyl alcohol as the benchmark reaction. The reaction was first carried out at room temperature in the presence of the catalyst 3 (2 mg, 0.000863 mmol with ChemistrySelect 2016, 1, 5929 – 5935

respect to Ru), benzyl alcohol (4 a, 1 mmol), H2O2 solution (1 mL) and TBAB (0.05 mmol) in DCM (3 mL). The reaction however results in very poor yield of the product (23 %) even after prolonged reaction time of 24 h. The reaction was then carried out at refluxing condition. It was observed that refluxing temperature significantly reduced the reaction time and also increased the yield of the product (89 %) within 2 h. In an attempt to establish the role of the catalyst, the benchmark reaction was set up without the catalyst and it afforded very low yield (3 %) of the product and the rest was the starting materials. This result clearly signifies that the catalyst 3 is an essential component of this transformation. Next, the role of the phase transfer catalyst (PTC) such as TBAB and TBAI were also investigated and the best result was obtained when TBAB 5931


Full Papers Table 1. Metalloprotein-inspired ruthenium catalyst (3) activity under various condition.

Entry

PTC

Catalyst 3 (mg)

Time (h)

Yield[a] (%)

1[b] 2[c] 3[c] 4[c] 5[c]

TBAB TBAB TBAB TBAI -

2 2 2 2

24 2 2 2 2

23 89 3 78 27

Reaction condition: Benzyl alcohol (4 a, 1mmol), PTC (0.05 mmol), H2O2 (1 mL) and ruthenium catalyst 3 in DCM (3 mL). a)Isolated yield after purification. b)At room temperature. c)under reflux.

Figure 2. Powder X-ray diffraction (XRD) data of the Ru-polymer complex after background correction. The inset figure is the background uncorrected XRD data.

was used as PTC (89 %, Table 1). The reaction was also performed in the absence of PTC but only 27 % yield was obtained. For checking the selectivity of the present protocol we continued the reaction for 24 h. To our delight it was observed that even upon prolonged reaction time of 24 h, the product 5 a was not over oxidized to its corresponding acid, justifying the selectivity of the present method. Inspired by the initial success next we decided to optimize the other reaction parameters.

The amount of catalyst loading required for the present reaction was tested and it showed that 2.0 mg (863 mol ppm w.r.t Ru) catalyst producing the best result. Higher amount of catalyst (> 2.0 mg) did not improve the yield of the product (Figure 4). The turn over number (TON) and turn over frequency (TOF) of the present catalyst (with respect to Ru) were calculated as reported in SI in 1031 and 516 h 1 respectively. Screening of solvent was also carried out for the present oxidation process. Several solvents such as Dioxane, Toluene, THF, H2O and DCM were investigated and maximum (89 %) yield was obtained in DCM. When the same reaction was performed under neat condition it afforded only 47 % yield. (Figure 5). After optimization of the reaction parameters, we investigated the versatility of the present protocol, likewise various alcohols were tested (Figure 6). Initially a variety of primary aromatic alcohol substrates with both electron donating and

Figure 3. (A, B) Represents the TGA and XPS data of the Ru-polymer complex.


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Figure 4. Effect of catalyst loading. Reaction condition: Benzyl alcohol (4 a, 1mmol), PTC (0.05 mmol), H2O2 (1 mL) and ruthenium catalyst 3 (various loading as indicated in graph) in DCM (3 mL) under reflux for 2 h.

Figure 6. Substrate scope.

corresponding ketone in moderate yield but aliphatic alcohol like butanol (4 o) did not produce the corresponding aldehyde, only starting material was witnessed. The results are summarized in the table below (Table 2).

Table 2. Metalloprotein-inspired ruthenium (3) catalyzed oxidation of alcohols.

Figure 5. Chart showing the effect of solvent for the present oxidation process. Reaction condition: Benzyl alcohol (4 a, 1mmol), PTC (0.05 mmol), H2 O2 (1 mL) and ruthenium catalyst (3, 2 mg) in solvent (3 mL) under reflux for 2 h.

electron withdrawing para-substituted groups were investigated (4b–e). All the alcohols were selectively oxidized to their corresponding aldehydes in good yield. Next o-substituted and m-substituted alcohols were also subjected to the present oxidation process. It was observed that in case of o-substituted alcohols (4f–h) good conversion to their corresponding product was achieved. However, a slightly lower yield was observed for m-substituted alcohols (5 i). The present protocol was then applied to secondary aromatic alcohols (4 j, 4 k and 4p–r); all the substrates resulted in good yield of the product. Allylic alcohol such as cinnamyl alcohol (4 l) was also put under investigation; only 31 % yield of product was obtained. Heteroaromatic system such as furfuryl alcohol (4 n) with the present method resulted in moderate yield of the corresponding aldehyde. Oxidation of cyclic aliphatic alcohol such as cyclopentanol (4 m) and cyclohexanol (4 s) also produced the ChemistrySelect 2016, 1, 5929 – 5935

Reaction condition: Alcohol (4a–s, 1 mmol), TBAB (0.05 mmol), H2O2 (1 mL) and metalloprotein inspired ruthenium catalyst (3, 2 mg) in DCM (3 mL) was refluxed for 2 h. Yield refers to isolated yield after purification. a) n.f = not formed.

Further, a gram scale reaction was also performed upon the benchmark reaction to highlight the industrial applicability of the present method. The reaction proceeded smoothly, however a slightly lower yield of the product (85 %) was obtained. Thus the present catalytic system, already demonstrated 5933


Full Papers Table 3. Gram scale reaction.

Reaction condition: Benzyl alcohol (4 a, 10 mmol), H2O2 (10 mL), TBAB (0.5 mmol), and ruthenium catalyst (3, 20 mg) in 30 mL of DCM was refluxed for 2 h.

beneficial to academia, show potential to be improved to be useful also for industries. For economic and environmental aspect, reusability is an important parameter of any good catalyst. Therefore the reusability of the catalyst was also investigated. After the completion of the reaction, the insoluble catalyst was then separated by means of simple filtration and washed with DCM, diethyl ether and dried. The separated catalyst was then reused for another set of reaction under the same experimental condition. It was observed that the catalyst could be reused for five consecutive runs without much depreciation in the yield of the product (Figure 7). SEM and EDX analysis of the reused

icant yield (3 %) of the product thus indicating negligible amount of leaching of the catalyst. To check the leaching more precisely, we carried out ICP-AES analysis after reaction. In a typical experimental set up, benzyl alcohol (4 a, 1 mmol), 1 mL of H2O2 solution, TBAB (0.05 mmol), catalyst 3 (2 mg) and 3 mL of DCM was refluxed with stirring for 2 h. After completion of the time mentioned, the reaction mixture was filtered. Then an ICP-AES analysis of the organic phase was conducted and it resulted in almost insignificant leaching (0.709 ppm of Ru) of the catalyst.

Conclusions In summary a highly stable, efficient and reusable metalloprotein-inspired ruthenium catalyst was developed via a molecular convolution method. The ruthenium catalyst serves as means of stable and efficient catalytic system in the oxidation process achieving the desired product within a short period of time. The present protocol highlights the reusability of the catalyst for five consecutive runs with the selectivity of the desired product. Therefore this method can be beneficial for academic purposes and implemented for industrial use.

Acknowledgements We thank the Department of Chemistry, DST (sanctioned no: SERC/F/0293/2012-13), Sophisticated Analytical and Instrumentation Facility (SAIF) of North-Eastern Hill University, IIT-Bombay, IASST-Guwahati and UGC for supporting this work under Special Assistance Programme (SAP) and DST-PURSE. Keywords: Metalloprotein inspired ruthenium complex oxidation · reusability · ppm level · catalysis and ruthenium

Figure 7. Reusability chart of the catalyst with error bars.

catalyst was also carried out and it showed almost negligible change in composition or general morphology of the catalyst (Figure 8). Five model reactions for each set under the same reaction condition were performed to access the reproducibility. The reproducibility of the reaction was then calculated using the mean data for each set (88.7, 88.3, 87.8, 87.4 and 86.6) and the error bars are reported by means of standard deviation method. The leaching test of the catalyst was also performed using a simple hot filtration test. Catalyst 3 (2 mg) and 3 mL of DCM was refluxed with stirring for 2 h. After the time mentioned, the reaction mixture was filtered, benzyl alcohol (4 a, 1 mmol), 1 mL of H2O2 solution and TBAB (0.05 mmol) was added and refluxed with stirring for 2 h. The reaction resulted in almost insignifChemistrySelect 2016, 1, 5929 – 5935

·

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Figure 8. (A, B) SEM and EDX image of the catalyst after use.

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Submitted: June 16, 2016 Accepted: October 31, 2016

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